I just adopted a shelter cat. She's about a year old. She keeps chasing and hitting my older male cat making him scream. Today they were wrestling on the floor and the older male got up and came over to me for protection. I don't know what to do. Seems from many of the discussions, I may have to bring her back to the shelter. Anyone have success in this situation?

Seems from many of the discussions, I may have to bring her back to the shelter.

What "discussions" are you referring too? I think most people here would suggest that you first go back to basics and properly re-introduce the cats to each other. If your cats are fighting, they need to be separated ASAP and kept separated while you gradually get them used to each others presence through scent-swapping and developing positive associations with each other. There are many websites dedicated to this topic but here are a couple to get you started: